Underwater pelletization equipment and its use following extrusion processing have been implemented for many years by Gala Industries, Inc. (“Gala”), the assignee of the present invention. Pelletization dies and die plates, in particular, are demonstrated in prior art disclosures including, for example, U.S. Pat. Nos. 4,123,207, 4,500,271, 4,621,996, 4,728,276, 5,059,103, 5,403,176, 6,824,371, 7,033,152, U.S. Patent Application Publication Nos. 20060165834 and 20070254059, German Patents and Applications including DE 32 43 332, DE 37 02 841, DE 87 01 490, DE 196 51 354, and World Patent Application Publications WO2006/081140 and WO2006/087179. These patents and applications are all owned by Gala and are expressly incorporated herein by reference as if set forth in their entirety.
As well understood by those skilled in the art, die plates used with rotating cutter hubs and blades, such as in underwater pelletizing, have the extrusion orifices or through die holes arranged in a generally circular pattern, or groups of multiple die holes arranged (as in pods or clusters) in a generally circular array. As so arranged, the rotating blades can cut the extrudate as it exits the die holes along a circular cutting face.
It is known in the field of plastic extrusion and cutting to feed plastic into a die plate for extrusion and solidification upon the exit from the die plate, and then to cut the extruded plastic into small pieces externally of the die plate. However, a known problem consists of the plastic freezing up within the die holes and either partially or completely blocking the passage of the plastic therethrough, with the resulting disruption of the entire operation.
To maintain the polymer at a sufficiently high temperature, insulation is desirable to reduce heat transfer from the extrusion die and the molten polymer being extruded through the extrusion orifices to the water circulating through the water box of the underwater pelletizer. Ineffective insulation can result in excessive cooling of the molten polymer as it is being extruded through the extrusion orifices causing freeze off of the molten polymer at the die face.
U.S. Pat. No. 4,378,964 and World Patent Application Publication No. WO1981/001980 disclose a multi-layer die plate assembly for underwater pelletization of polymeric materials in which an insulation layer, preferably zirconium oxide, is fixedly positioned between the body of the die plate and the layers comprising the cutting face of the die. Adjacent or proximal to the insulation layer is a heating chamber through which is circulated a heating fluid for maintenance of the temperature of the die.
U.S. Pat. No. 4,764,100 discloses a die plate construction specifically described for underwater pelletization of plastic extrudate including a closed insulating space formed between the baseplate and the cutting plate through which penetrates the extrusion nozzles, and optional inserts serve to further strengthen and support the structure.
Vacuum heat insulating cavities between extrusion nozzles are disclosed in U.S. Pat. No. 5,714,713 in a multi-step process that includes electron beam welding while the die components are maintained under high vacuum. This disclosure is extended to vacuum heat insulation portions in areas peripherally external to the extrusion nozzles for enhanced insulation performance in U.S. Pat. No. 5,989,009.
Similarly, closed continuous thermal stabilization cavities filled with air or gas are disclosed in U.S. Pat. No. 6,976,834. Additionally, brazing in a furnace at high temperature, 900° C. to 1200° C., under vacuum is disclosed as a manufacturing process with controlled cooling under argon to prevent oxidation thusly presenting an opportunity to introduce vacuum into the thermal stabilization cavities.
German Patent Application No. DE 100 02 408 and German Patent Utility Model No. DE 200 05 026 disclose a hollow space or a multiplicity thereof in the inner region of the nozzle plate and the nosecone extension to enhance temperature control by virtue of the reduction of mass necessitating temperature maintenance and thusly introducing thermal insulation. Use of solid, liquid, or gas as insulating materials is disclosed therein.
World Patent Application Publication No. WO2003/031132 discloses the use of ceramic plates for insulation of the die face from the heated portion of the die body.
Finally, Austrian patent application AT 503 368 A1 discloses a thermally insulated die plate assembly with a detachable face plate that is sealed to the discharge end of the extrusion orifice nozzles by an O-ring or metal seal. This die plate assembly is very fragile and highly susceptible to process melt leakage, thus requiring considerable maintenance.
There is, therefore, a need for a thermally insulated die plate assembly which is robust in construction, retains the air pocket in a sealed condition, requires low maintenance and provides high performance.